The present invention relates to a linear electrical machine and in particular linear electrical generator for a free piston engine generator.
In standard combustion engines, pistons are mechanically restrained within their cylinder as a result of being connected to a crankshaft, which is driven rotationally as a result of the reciprocating linear movement of the piston within the cylinder. In a free piston engine, however, the piston is not connected to a crankshaft, although pistons may be provided within an engine of this type that do have external mechanical linkages such as taught in U.S. Pat. No. 7,383,796.
Furthermore, it is known that electrical power can be generated by movement of a reciprocating piston in a free piston engine through one or more electrical coils to generate a magnetic flux change, for example U.S. Pat. No. 7,318,506. In this arrangement the piston carries a first coil and as it reciprocates within the cylinder it generates an electric current in a second coil that surrounds the cylinder. However, the piston is constructed from a solid piece of material that is permeable to magnetic flux and is necessarily very short relative to the length of the cylinder so that it may induce the flux changes as it passes through the second coil.
In existing free piston engines, the length of the piston is typically less than five times the diameter of the cylinder bore of the combustion chamber. The power output of the electrical machine in a free piston engine is determined by the area of the air gap, and to achieve an air gap area sufficient for a given combustion chamber geometry, which is determined by the diameter and swept volume, the diameter of the electrical machine is generally larger than the diameter of the combustion chamber. This change in diameter necessitates complex and expensive mechanical solutions to seal each combustion chamber, and to ensure that these are coaxially aligned with each other and with the axis of the intervening electrical machine.
Three fundamental design challenges for free piston engine generators are:                firstly to achieve high electrical efficiency in an electrical machine, sized appropriately for the combustion power output of the engine        secondly to achieve a good degree of piston motion control and combustion chamber design so that the combustion process efficiency and completeness can be assured        Thirdly, to resolve the first two challenges at the lowest possible cost        